During travel on dusty roads or in rainy or snowy weather conditions, dirt, such as dust and mud, tends to collect very rapidly on the rear light units of motor vehicles. This dirt considerably reduces the brightness of the light units, making it more difficult for them to be detected from following motor vehicles.
In order to remedy this problem, whih is particularly apparent in motor vehicles with a substantially flat rear panel, mechanical light-wipers substantially similar to those fitted to the front light units of some motor vehicles could be adopted, in principle.
Some manufacturers have also suggested that the optical surfaces of rear light units be made with a particular stepped conformation which tends to reduce the effect of dirt to some extent.
Up to now, however, no solution has been suggested which enables the dirtying of the rear light units of motor vehicles to be prevented completely without giving rise to problems in terms of complexity and cost.